Conventional high electron mobility transistors (HEMTs) fabricated in GaN/AlGaN technology are typically majority carrier devices, i.e. only one type of carriers (electrons or holes) are mainly involved in determining the electrical properties of the device. This feature is particularly beneficial for III-V diodes when compared to classical Silicon body diodes. With Silicon body diodes, the main electrical properties are controlled via the doping and device dimensions, and both the minority and majority carriers affect device operation. In particular, during switching operations conventional silicon devices experience a so-called “reverse recovery time” which represents a time delay needed to remove the stored charges during the device conduction before the device can enter blocking mode operation. This reverse recovery time delay is particularly detrimental because it highly increases switching losses. For this particular reason, Schottky diodes and in general majority carrier devices, are preferred in applications where higher losses due to diode reverse recovery time cannot be tolerated.
GaN/AlGaN technology represents a large step forward in power electronics due to higher performance in terms of current drive capability, breakdown strength, and switching frequency to name a few, when compared to conventional silicon technology. One key feature of conventional present-day GaN/AlGaN technology, which is a disadvantage for forming integrated diodes, is the difficulty in accurately controlling and activating doping in the device. Indeed, the main electrical properties of a typical GaN/AlGaN device are controlled via polarization charges which allow the device performance to be tailored without the use of doping. The difficulty in controlling doping in III-V materials (especially p-type doping) represents a major drawback in designing high-blocking mode and low forward bias diodes.
With a typical GaN/AlGaN HEMT there is no real body diode, as is conventionally present in a typical silicon device. However, when the HEMT device is in an off-state condition, i.e. in the absence of an electron channel below the gate electrode, a pseudo-body diode can be observed which connects the source terminal to the drain terminal. An electron in the channel of the pseudo-body diode must overcome an energy barrier having a certain height in order to traverse from the source to drain electrode. Different from conventional silicon technology, in GaN technology this barrier height is not fixed via the doping profile but by the material properties (energy gap) and also by the voltage applied to the gate electrode. The stronger the device is biased in pinch-off, the higher the forward bias of the pseudo-body diode.
When a negative voltage is applied to the drain electrode, with the device being in off-state conditions, the conduction band on the drain side is pulled up and therefore the effective barrier height that carriers must overcome to reach the source electrode is lowered. When a certain threshold voltage is reached, the pseudo-body diode opens and allows current flow between the source and drain electrodes. However, typical forward bias voltages for such a pseudo-body diode in GaN technology is in the order of 3V and increases (becomes more negative) when the device is biased more strongly in off-state conditions. This particular characteristic of the pseudo-body diode negatively impacts the switching behavior of the power transistor in all applications where a low forward bias body diode is needed. In some conventional approaches, a lateral Schottky diode with a low forward bias is integrated with the HEMT. This approach however results in an area penalty due to integration of a series lateral device. Also, the pre-existing GaN baseline process must be modified to include a low forward bias Schottky diode which increases cost. Furthermore, the Schottky diode can become unstable due to surface effects.